Sometimes, a bulb has two functions. Usually if this happens to me, a changing of the bulb fixes it, as the pieces inside can go bad, leading to the condition you describe I've had bulbs that are used as turn signals, brakes, and parking lamps go bad. Sometimes, two out of the three functions will work, or one, or none! Changing the bulb is cheap, and easy. You may want to look in that direction.
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